Abstract Smoking and tobacco use are major risk factors for many diseases including lung and head and neck (H&N) cancer. A long term goal is to establish susceptibility markers for lung and H&N cancer to identify tobacco users at higher risk for these cancers. UDP-glycosyltransferase (UGT) enzymes play a critical role in the detoxification of many carcinogens abundant in tobacco and/or tobacco smoke including polycyclic aromatic hydrocarbons (PAHs) and tobacco-specific nitrosamines (TSNAs). Preliminary data suggest that members of the minimally-studied UGT2A and 3A sub-families of metabolizing enzymes are expressed in tobacco target tissues including lung and H&N, exhibit activity against tobacco carcinogens, and that polymorphic and splicing variants exist that alter their activity, thus potentially altering local detoxification of tobacco carcinogens at these targt sites. These characteristics suggest that UGT2A and 3A enzymes are potentially important susceptibility markers for tobacco-related cancers. The goal of this proposal is to test our hypothesis that UGT2A and 3A activity is important in the local detoxification of tobacco carcinogens in tobacco target tissues, and that UGT2A and 3A activities and expression are altered via genotypic and splicing mechanisms and play a role in risk of tobacco-related cancers. The specific aims of this proposal are to, (1) Characterize the expression and activity of UGT2A and 3A enzymes; (2) Examine the importance of UGT2A and 3A SNPs to risk for cancers of the lung and H&N; and (3) Examine UGT2A and 3A splicing mechanisms as a form of UGT2A and 3A regulation by determining whether UGT2A and 3A splice variants function to regulate UGT conjugating activities and potentially act to alter tobacco-related cancer risk. The proposed studies will enable us to better understand the potential role of the UGT2A and 3A enzymes as susceptibility markers for tobacco- related cancer induction, help us identify subjects for targeted prevention strategies, and enable us to evaluate the innovative concept that differential splicing may act as a form of gene regulation that plays a role in cancer risk.